Double data rate (DDR) memory is a type of dynamic random access memory (DRAM) which transfers data on both the rising edge and the falling edge of the clock. DDR1-DDR4 refer to different generations of DDR technology with DDR4 being the most recent. Generally, each new generation may improve power consumption or data rate.
Current ubiquitous DDR memory interfaces have strict timing requirements for how long memory can take to respond to normal memory operations, such as read or write. These timing requirements leave little opportunity to perform any operations that may take a longer time than a few nanoseconds. If a longer time is taken, the processor may generate a non-maskable interrupt (NMI) to the operating system to indicate a potential memory error/failure.